Rosin-based agents have been used widely as sizing agents for some time. It has been known that the size development of the rosin-based sizing agents is due to the fact that the aluminium sulfate used as an assistant acts as a yield and a hydrophobic agent for the rosin-based sizing agents. Since this aluminium sulfate dissociates and shows acid, rosin-based sizing agents have been used in the acid range or zone.
However, in these days, a problem of durability exists for acid paper, and thus calcium carbonate is widely used as a coat color pigment for printing paper, thereby the amount of calcium carbonate contained within waste paper increases, resulting in the trend of making paper in the neutral range.
The conventional rosin-emulsion sizing agent uses mainly fortified rosin modified with an .alpha.,.beta.-unsaturated dibasic acid as an anionic surfactant and the sizing effect thereof decreases remarkably in the system described above, and particularly in the range above pH 6.5 for papermaking. The amount of sizing agent used must be increased so as to obtain the desired sizing level, thereby costs rise due to the excess amount of size, and disadvantages on operation occur such as foaming and deposition of pitch in the papermaking. Such factors have a bad influence on the nature of the prepared paper. Even if the amount of addition is increased in the range of pH 7.5 or more, satisfactory sizing property cannot be obtained in comparison with cellulose reactive sizing agents such as alkyl ketene dimer-based and alkenyl succinic acid anhydride-based sizing agents. AKD and ASA are used as dispersants whose protective colloid is cation starch and so on, however, the stability of these dispersant type of reactive sizing agents is bad. If they are accumulated in the papermaking, the stickiness increases with the destruction of the dispersant and big problems such as staining of the papermaking machines occur on operation, therefore, an improvement is required.
As described above, the improvement of AKD- and ASA-based sizing agents is being examined and recently, a rosin-based neutral sizing agent has been proposed. For example, it is known that Japanese Patent Tokkaisho 62-250297, 63-120198, Tokkouhei 2-36629 disclose the proposition.
Japanese Patent Tokkaisho 62-250297 discloses the reaction product of rosins, polyhydric alcohol consisting of C, H and a O and .alpha.,.beta.-unsaturated carboxylic acid derivative, and that the sizing effect decreases remarkably during the sizing above pH 7 of in a papermaking system, therefore, it being not necessarily a satisfactory sizing agent as a sizing agent for neutral paper.
On the other hand, the invention described in Japanese Patent Tokkaihei 2-36629 is characterized in that the sizing property around a neutral range due to the reaction of a partial amino alcohol ester of rosin with an .alpha.,.beta.-unsaturated dibasic acid is superior to the sizing property around a neutral range to polyhydric alcohol ester disclosed in 62-250297. However, a good emulsion cannot be obtained and as for the sizing property around neutral range, it cannot be said to be a satisfactory sizing agent.
The invention described in said Japanese Patent Tokkaisho 63-120198 is a rosin-based emulsion sizing agent comprising fortified rosin, methacrylic alkylester and/or copolymer of styrene-compound and methacrylic alkylaminoalkylester or methacrylic alkylaminoalkylamide, however, this agent has difficulty in the size in the neutral range and is not a satisfactory sizing agent for neutral paper.
Moreover, in Japanese Patent Tokkaisho 63-40312 and Tokkaihei 4-91292, copier paper for copying machine, using calcium carbonate as a filler and alkenylsuccinic acid anhydride as a sizing agent has good properties, but the rosin-based sizing agent is not suitable for copier paper for copying machines. This is because the conventional rosin-based sizing agent does not show the size in a pH range (weak acid range) for papermaking using much calcium carbonate and said paper is made under the conditions (in weak acid to acid range) using talc as a filler.
Then, in view of this situation, the object of the invention is to provide a better sizing agent with good stability, particularly which is rapid in getting started in the neutral range or zone, and a method of producing the same.